Paired helical filaments (PHFs) and straight filaments (SFS) make up the neurofibrillary tangles (NFTs) of Alzheimer's Disease (AD) and other neurofibrillary degenerative conditions, where their occurrence is well correlated spatiotemporally with cell loss and dementia. The cellular mechanisms and consequences of tau filament formation are critical pathogenic factors in neurodegenerative conditions featuring NFTs, since neurofibrillary degeneration (NFD) has been shown to be induced by mutations in the tau gene. In AD, it is likely that NFT formation and neurodegeneration are influenced by abnormalities in the metabolism of amyloid precursor protein (APP), which are thought to produce the extracellular beta-amyloid (Abeta) deposits of AD, and initiate AD pathogenesis. However, murine transgenic models which generate mutant APP and/or (Abeta) deposits do not develop tau filaments or NFD, and recently developed tau overexpression transgenics (which do develop NFD) impose significant technical limitations on the elucidation of the cellular mechanisms underlying tau-induced NFD in a given cell type. There is presently only one in situ neuronal model of tau-induced NFD in which the role of extracellular deposits of (Abeta) or any other factor that might affect tau filament formation (such as tau hyperphosphorylation, the presence of intracellular Abeta 1-42, or the binding of heparin to tau) can be readily studied in detail at the single cell level. This is a set of giant identified neurons (ABCs) in the lamprey CNS, which was developed as an NFD cellular model by the PI during the first funding period of this grant. In ABCs, overexpression of human tau via intracellular plasmid injection causes tau filament formation and accumulation, dendritic microtubule loss, synapse loss and eventually neuronal degeneration. The goal of this renewal is to use the ABC cellular model to test directly the relationships between the extent and type of tau filament formation, the state of tau phosphorylation, and the loss of dendritic MTs and synapses caused by human tau expression in vivo by manipulating the following factors during the expression of exogenous human tau in ABCs: C terminal deletions of htau that accelerate or block filament formation in vitro Mutations in tau that have been directly linked to NFD in humans Intracellular levels of heparan sulfate proteoglycans and Abeta 1-42 and extracellular Abeta during htau expression The results of this study should indicate how these manipulations are related to the development of neurofibrillary changes resembling those seen in Abeta and familial tauopathies in single, identified vertebrate central neurons in vivo. Consequently, the proposed studies should lead to fundamental new insights into the cellular mechanisms connecting tau metabolism with NFT formation, the presence of Abeta deposits and the cytopathology of AD and other forms of NFD that are presently unavailable from studies in cell culture or murine transgenic systems.